The technique of lining metal pipes with an elastomeric material to permit the use of such pipes for conveying acidic caustic, corrosive or abrasive liquids is well-known in the art, including the lining of such pipes for use in conjunction with segmented pipe couplings.
A segmented pipe coupling is comprised of arcuate coupling segments that contain and confine an elastomeric seal, and, which are secured to each other at adjacent mutually-presented ends of the respective coupling segments by traction bolts. On tightening down of the traction bolts, the traction bolts act to draw the coupling segments into engagement with the ends of a pipe or fitting, and, in so doing, act to compressively engage the contained elastomeric seal with the periphery of the pipe ends.
While reference is made to pipes throughout this disclosure, it will be understood that the pipe can be comprised of a nipple of a flanged coupling, or, a nipple of any other type of fitting, such as a tap, valve, Y- or T-, or any other type of fitting that provides a nipple to be coupled to an adjacent pipe end by means of a segmented pipe coupling.
In one method, the lining of such pipes is accomplished by inserting a quantity of an uncured plastisol of polymeric material into the bore of the pipe, and, then rotating the pipe to spread the liquid polymer evenly throughout the pipe bore, either under the influence of gravity, or, under the influence of centrifugal action.
During the rotation of the pipe, the elastomeric material is permitted to cure, and thus form a continuous coating or lining on the pipe interior of a material that is impervious and chemically non-reactive to the liquid to be conveyed by the pipe.
End dams commonly are employed in such an operation, the end dams being operative to cast a coating of the elastomeric material onto the radially extending end surfaces of the pipe, and, in some instances, also to cast a layer of the elastomeric material onto the exterior of the pipe ends at positions adjacent to the respective ends thereof.
This latter type of casting procedure is the one that commonly is employed when the pipe is intended for use with segmented pipe couplings.
The more common method of elastomeric lining uses sheet stock positioned in the pipe bore, stretching it over the radially extending pipe end, and onto the outside diameter. The elastomer on the outside diameter is sized and shaped by mechanical buffing, or knives, either manually or by machine or tool.
When using segmented pipe couplings to secure the adjacent ends of pipes to each other, there is no guarantee that a hermetic seal will be formed at the radially extending end faces of the pipes. Thus, provision must be made to hermetically seal the joint at the outer periphery of the pipe ends.
This is accomplished by a folding back of the lining onto the outer periphery of the pipe ends for a distance sufficient that the lining on the pipe exterior will provide intimate sealing contact with the elastomeric gasket contained within the segmented pipe coupling. A seal is thus formed at the pipe exterior, and which is entirely independent of any sealing action produced at the radially extending end faces of the pipes.
Commonly, pipes, including lined pipes, are provided with either cut or rolled grooves adjacent their ends, in order to accommodate the keys of the respective coupling segments. Also, commonly, the keys are required to bottom down in the grooves, thus to provide the required clamping action of the segmented pipe coupling onto the pipe ends. This consideration, in turn, determines the required internal diameter of the sealing gasket, which must be of a standard size for any particular standard size of coupling.
If the folded-back lining on the exterior of the pipe extends beyond the outer diameter of the pipe, then, this will require a non-standard sealing gasket having an internal diameter that is greater than that of the internal diameter of a standard sealing gasket. Not only is the provision of a non-standard gasket an inconvenience in warehousing of the couplings, but, there is a danger that a careless workman will assemble a non-standard gasket into the coupling, when, in fact, a standard gasket is required, the two versions of the gaskets differing in visually imperceptible respects.
If a non-standard gasket is assembled into a standard segmented pipe coupling, the probability is that insufficient compressive force will be applied to the gasket upon tightening down of the coupling to insure against leakage axially of the exterior surface of the pipe ends.
Knowing of this problem, rather than to have the folded-back lining on the exterior of the pipe ends extend beyond the outer diameter of the pipes, it has been common practice to machine the pipe ends to a reduced external diameter, thus to accommodate the folded-back lining on the pipe exterior. The folded-back lining then can be arranged to have an external diameter identical with the exterior diameter of the pipe end, thus removing the necessity for a sealing gasket of non-standard dimensions. However, such a machining operation represents added cost in the manufacture of a piping system, and may reduce the mechanical strength of the pipe.